1. Field of the Invention
The present invention generally relates to self-attaching fasteners and, more specifically to clinch nuts.
2. Description of the Related Art
Self-attaching fasteners are used in many industries such as, for example, the automotive and appliance industries to secure various components to metal panels. When clinch nuts are attached to the metal panels, screws or bolts are threaded into the clinch nuts and tightened to prescribed torque values. During installation, the clinch nuts must have sufficient rotational resistance to keep them from rotating relative to the metal panels when the screws are inserted and tightened. During service, the clinch nuts must have sufficient push-out resistance to keep them from pulling out of the metal panel when external forces such as, for example, vibration or other tensile forces are applied.
A clinch nut typically includes a central punch or pilot portion which at least partially extends into an opening in a metal plate or panel. When the clinch nut is self piercing, the central pilot portion cooperates with tooling to form the opening in the metal panel. The clinch nut is attached to the metal panel using a supporting die member which forms a mechanical interlock between the clinch nut and the metal panel. The die member typically deforms the metal panel into an annular groove extending around the clinch nut's pilot or punch portion and/or deforms the pilot or punch portion of the clinch nut over the metal panel to entrap the metal panel.
For example, U.S. Pat. No. 3,053,300 discloses a clinch nut having a central pilot portion which extends through a pre-formed opening in a metal panel and is folded over to stake the periphery of the opening. The deformation of the central pilot forces the metal panel to conform to an undulating surface of the annular groove and to form the interlock between the clinch nut and metal panel. While this clinch nut may have a relatively high push-out resistance, the deformation of the central pilot can easily distort the internal threads of the clinch nut.
One approach to eliminate distortion of the internal threads when deforming the pilot is to deform the metal panel to form the interlock rather than the pilot of the clinch nut. For example, U.S. Pat. Nos. 3,878,599 and 4,690,599 each disclose a clinch nut having an undercut on either the inner or outer wall of the groove. Material of the metal panel is forced into the undercut to improve the interlock formed between the clinch nut and the metal panel. With relatively thin metal panels, however, very little material is forced into the undercut, resulting in a relatively low push-out resistance.
One approach to increase the push-out resistance of clinch nuts of this type is to form a double-undercut groove. For example, U.S. Pat. No. 5,340,251 discloses a clinch nut having undercuts in both the inner and outer walls so that the annular groove is “dove-tail” shaped in cross section. The metal panel is forced into both of the undercuts to form an improved interlock between the clinch nut and metal panel. The deformation of the metal panel required to fill both undercuts, however, is difficult to obtain using conventional forming techniques, resulting in inconsistent push-out resistance.
An additional problem with the above-noted self-clinching fasteners is that they typically do not function well with thin metal panels. Accordingly, there is a need in the art for an improved clinch nut which can be reliably and consistently attached to a thin metal panel having sufficient push-out strength, sufficient rotational resistance, and without having distortion of the internal threads. Additionally, there is a need for an improved die member for installing a clinch nut in a thin metal panel having sufficient push-out strength, having sufficient rotational resistance, and without having distortion of the internal threads. Furthermore, there is a need for both the clinch nut and the die member to be relatively inexpensive to produce and relatively easy to use.